1. Field of the Invention
The present invention relates to a compound for organic electroluminescent (EL) devices and an organic electroluminescent device using the same, and more particularly, to a compound for organic electroluminescent devices and an electroluminescent device using the same with a high luminous efficiency, reduced driving voltage, high thermal resistance and long lifetime.
2. Description of Related Art
There has been an increasing interest in developing novel organic materials that cater to organic light emitting device (OLED) applications. Such devices are commercially attractive because they offer the cost-advantageous fabrication of high density pixeled displays exhibiting brilliant luminance with long life times, high efficiency, low driving voltages and wide color range.
A typical OLED comprises at least one organic emissive layer sandwiched between an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton” which is a localized electron-hole pair having an excited energy state is formed. Light is emitted when the exciton relaxes through a photoemissive mechanism. To improve the charge transport capabilities and also the luminous efficiency of such devices, additional layers around the emissive layer, such as an electron transport layer and/or a hole transport layer, or an electron blocking and/or hole blocking layer(s) have been incorporated. Doping the host material with another material (guest) has been well demonstrated in literature to enhance the device performance and to tune the chromaticity.
The initial OLEDs used emissive materials that emitted light from their singlet states, termed as “fluorescence”. Fluorescent emission generally occurs in a time frame of less than 10 nanoseconds. Several OLED materials and device configurations utilizing fluorescence are described in U.S. Pat. No. 4,769,292, U.S. Pat. No. 5,844,363, and U.S. Pat. No. 5,707,745, which are incorporated herein by reference in their entirety.
More recently, OLEDs having emissive materials that emit light from triplet states (phosphorescence) have been demonstrated in literature, Nature, 1998, No. 395, p. 151 and Appl. Phys. Lett., 1999, No. 3, p. 4, and U.S. Pat. No. 7,279,704, which are incorporated herein by reference in their entirety.
Selection of a host material in phosphorescent OLED's is difficult especially since the non-emissive triplet excited state of the host material must be higher than that of the guest phosphor (dopant). In addition, a host material must have good charge transport properties for an efficient organic EL device.
JP2001-313178 disclosed CBP (4,4′-bis(N-carbazolyl)-1,1′-biphenyl) as the host material, which is characterized by having a good hole transport property but poor electron transporting. Hence the use of CBP as a host material for tris(2-phenylpyridine) iridium (hereinafter referred to as Ir(ppy)3), a green phosphorescent emitter, disturbs balanced injection of electrical charges, causing excess holes to flow towards the electron transporting layer, thereby decreasing the luminous efficiency. Moreover, due to its low molecular weight, it tends to crystallize and thus is not suitable for OLED devices.
One of the means to solve the above problem is to introduce a hole blocking layer between the light emitting layer and the electron transporting layer as described in JP2002-305083. This hole blocking layer accumulates holes efficiently in the light emitting layer and contributes to increase the probability of recombination of holes and electrons and thus enhances the luminous efficiency. Currently, the hole-blocking materials in general use include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and phenylphenolato-bis(2-methyl-8-quinolinator-N1,08)aluminum (hereinafter referred to as BAlq). However, BCP tends to crystallize even at room temperature and lacks reliability as a hole blocking material and the life of the device is extremely short; whereas BAlq has insufficient hole blocking ability.
For a high luminous and efficient OLED's, a host material must have non-emissive high triplet energy and a balanced electrical charge (hole/electron) injection/transport characteristics. Moreover, the host material should also possess good electrochemical stability, high thermal resistance and excellent thin film stability. However, compound capable of satisfying all the said properties from practical considerations have not been known till date.
Attempts have been made to introduce molecular moiety that has an excellent hole transport property as represented by a carbazole or triarylamine and another moiety that has an excellent electron transport property as represented by pyrimidine or triazine into one and the same molecular skeleton, as phosphorescent host materials, as disclosed in the patent documents WO2003-78451, WO2005-76668, US2006-51616, JP2008-280330, WO2008-123189 and JP 2009-21336.
When a plurality of skeletons differing from one another in the electrical charge transport properties are introduced into one and the same molecule, the molecule may undergo large changes in the balance of electrical charges, leading to higher driving voltages, reduced lifetime and low efficiency.
Hence, there is a need to develop a material for an organic electroluminescent device for producing an organic electroluminescent device with a high luminous efficiency, reduced driving voltage, high thermal resistance and long lifetime.